JP7516760B2 - Water-resistant gas barrier film and method for producing water-resistant gas barrier film - Google Patents

Description

The present invention relates to a gas barrier film, and more particularly to a water-resistant gas barrier film and a method for producing the same.

Conventionally, various packaging materials with gas barrier properties such as oxygen barrier properties have been developed. In recent years, many gas barrier films have been proposed in which a metal oxide film such as silicon oxide or aluminum oxide is provided on a plastic substrate at a nanoscale.

However, forming a metal oxide film usually requires a vacuum film-forming apparatus and related manufacturing facilities for carrying out a deposition method, an ion plating method, or the like, and is therefore expensive in terms of cost.

In recent years, packaging materials are needed worldwide, and inexpensive barrier materials that can be processed by printers are needed to supply packaging materials at low cost. Polyvinylidene chloride has been used to achieve oxygen barrier properties, but its usage is decreasing due to the generation of harmful substances during incineration. Ethylene vinyl alcohol film is also widely used for packaging, but it is expensive.

In view of this, for example, a method of coating a thin film of a material using polyvinyl alcohol and an inorganic layered mineral has been proposed, as in Patent Document 1. While this composition provides excellent gas barrier properties, further study is required to obtain water-resistant adhesion.

International Publication No. WO 2013/129520

In view of the above problems, the present invention aims to provide a new gas barrier film having both oxygen barrier properties and water resistance, and a method for producing the water-resistant gas barrier film.

A water-resistant gas barrier film according to one aspect of the present disclosure is a gas barrier film having at least a gas barrier layer provided on a base film, the gas barrier layer being formed from a coating liquid containing (a) at least one of carboxy-modified polyvinyl alcohol and partially saponified polyvinyl alcohol, (b) fully saponified polyvinyl alcohol, and (c) polyethyleneimine.

In one embodiment, the degree of polymerization of the partially saponified polyvinyl alcohol and the fully saponified polyvinyl alcohol may be 3000 or less.

In one embodiment, the content of polyethyleneimine may be 10 mass % or less based on the total amount of the gas barrier layer.

In one embodiment, the polyethyleneimine may have a weight average molecular weight of 500 or more.

In one embodiment, the carboxy-modified polyvinyl alcohol may be a saponification product of a copolymer of a vinyl ester monomer and a carboxy-group-containing monomer.

In one embodiment, the water-resistant gas barrier film may be a sequentially stretched film.

A method for producing a water-resistant gas barrier film according to one aspect of the present disclosure includes the steps of: applying, onto a substrate film, a coating liquid containing (a) at least one of carboxy-modified polyvinyl alcohol and partially saponified polyvinyl alcohol, (b) fully saponified polyvinyl alcohol, and (c) polyethyleneimine; and heating and drying the applied coating liquid to form a coating film on the substrate film.

In one embodiment, the method for producing a water-resistant gas barrier film may further include a step of stretching the substrate film together with the coating film.

In one embodiment, the stretching ratio in the stretching step may be 2 to 20 times.

According to the present invention, a new gas barrier film having both oxygen barrier properties and water resistance can be provided. According to the present invention, a method for producing the water-resistant gas barrier film can also be provided. By using a coating liquid containing polyvinyl alcohol and polyethyleneimine, in which the polyvinyl alcohol is at least a part of a carboxy-modified polyvinyl alcohol or a partially saponified polyvinyl alcohol, a gas barrier film having water-resistant adhesion can be provided. The gas barrier film of the present invention having a stretchable polyvinyl alcohol-based coating film is inexpensive as a packaging material and can be processed by a printing machine or the like.

FIG. 1 is a schematic cross-sectional view showing a water-resistant gas barrier film according to an embodiment.

<Water-resistant gas barrier film>
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view showing a water-resistant gas barrier film according to one embodiment. The water-resistant gas barrier film 10 includes a gas barrier layer 2 on a base film 1 for imparting gas barrier properties to the base film. The gas barrier layer 2 is composed of specific components and has excellent water resistance.

(Base film)
Examples of the base film 1 include films made of, for example, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyamide, polypropylene, polycarbonate, cycloolefin, etc., but are not limited thereto. These materials may be amorphous. The base film may be a stretched film or a non-stretched film. However, if the base film is a stretched film, it is preferable because it has excellent mechanical properties such as tensile strength and heat resistance. The base film may be a film stretched only in one axis direction (for example, MD direction) as necessary.

The thickness of the base film 1 is not particularly limited, but it is preferably a general thickness of about 3 μm or more and 100 μm or less.

(Gas Barrier Layer)
The gas barrier layer 2 contains polyvinyl alcohol and polyethyleneimine, and the polyvinyl alcohol contains at least a portion of carboxy-modified polyvinyl alcohol or partially saponified polyvinyl alcohol. That is, the gas barrier layer 2 is formed from a coating liquid containing (a) at least one of carboxy-modified polyvinyl alcohol and partially saponified polyvinyl alcohol, (b) fully saponified polyvinyl alcohol, and (c) polyethyleneimine. More specifically, the gas barrier layer 2 can be said to be a layer obtained by heating and drying the coating liquid (a heat-dried product).

Generally, a coating film made only of polyvinyl alcohol (fully saponified) containing only hydroxyl groups can adhere to the substrate through ionic bonds, but the bond is prone to peeling due to the involvement of water. Therefore, it is necessary to introduce a substance with other functional groups to provide a strong bond with the substrate, but simply adding a material that easily bonds with hydroxyl groups leads to precipitation and a reduced pot life.

As a means to solve the problem, the inventors decided to use carboxy-modified or partially saponified polyvinyl alcohol in addition to fully saponified polyvinyl alcohol. They thought that by bonding the coating film to the substrate via the carboxy or acetate groups, it would be possible to obtain strong adhesion while maintaining the inherent polyvinyl alcohol properties of the film. They found that by using polyethyleneimine to bond the coating film to the substrate and by interposing polyethyleneimine between the carboxy or acetate groups and the substrate, a strong bond between the coating film and the substrate could be obtained, and thus completed the present invention.

The base film 1 and/or the gas barrier layer 2 of the water-resistant gas barrier film may contain additives such as antistatic agents, ultraviolet absorbers, plasticizers, and slipping agents, as necessary. Furthermore, at least the surface of the base film 1 on the side of the gas barrier layer 2 may be subjected to a modification treatment such as a corona treatment, a frame treatment, a plasma treatment, or an adhesion enhancing treatment. Furthermore, the water-resistant gas barrier film 10 may further include a functional layer such as a printed layer, a sealant layer, an adhesive layer, or a protective layer, depending on the application of the film.

The thickness of the gas barrier layer 2 is preferably 0.01 to 20 μm. If the thickness is less than 0.01 μm, the barrier properties are likely to decrease, and if the thickness is more than 20 μm, drying during layer formation is likely to be insufficient, and the suitability for winding coating may be poor.

(Polyvinyl alcohol)
As the polyvinyl alcohol, a general polyvinyl alcohol can be used. Polyvinyl alcohol can be obtained by polymerizing a vinyl ester monomer and then saponifying it. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl laurate, and vinyl stearate, but vinyl acetate is generally used.

The polyvinyl alcohol may be a copolymerized or post-modified modified polyvinyl alcohol. The copolymerized modified polyvinyl alcohol is obtained by copolymerizing a vinyl ester monomer with another monomer (unsaturated monomer) copolymerizable with the vinyl ester monomer, followed by saponification. The post-modified polyvinyl alcohol is obtained by copolymerizing a vinyl ester monomer, followed by saponification, and then copolymerizing the polyvinyl alcohol obtained by polymerizing the vinyl ester monomer with another monomer in the presence of a polymerization catalyst.

Examples of such other monomers include olefins such as ethylene, propylene, and isobutylene; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; vinyl chloride, vinylidene chloride, polyoxypropylene, and polyoxypropylene vinylamine.

In addition, when obtaining a carboxy-modified polyvinyl alcohol, such other monomers, i.e., carboxy-containing monomers, include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, itaconic acid, and monoesters of these carboxylic acids. As a raw material for the carboxy-modified polyvinyl alcohol, a combination of vinyl acetate and a carboxy-containing monomer is preferable. The modification amount in the carboxy-modified polyvinyl alcohol (the content of the carboxy-modified monomer unit in the total monomer unit (100 mol%) contained in the carboxy-modified polyvinyl alcohol) is not particularly limited, but can be 1 to 50 mol%.

A polymerization catalyst is used for polymerizing the raw material monomers. Examples of the polymerization catalyst include radical polymerization catalysts such as 2,2-azobisisobutyronitrile, benzoyl peroxide, and lauroyl peroxide. The polymerization method is not particularly limited, and bulk polymerization, emulsion polymerization, solvent polymerization, and the like can be used.

After polymerization, the polymer of the raw material monomer is subjected to a saponification step. The saponification can be carried out by dissolving the polymer in a solvent containing alcohol and using an alkali or acid, but it is preferable to use an alkali from the viewpoint of the saponification rate. Examples of the alkali include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, and alkali metal alkoxides such as sodium ethylate, potassium ethylate, and lithium methylate. The saponified product (polyvinyl alcohol) after saponification is dried and can be obtained as a powder.

The degree of saponification of the fully saponified polyvinyl alcohol is 98 mol% or more, but may be 99 mol% or more. When the degree of saponification of the fully saponified polyvinyl alcohol is 98 mol% or more, sufficient barrier properties are easily obtained. Furthermore, the degree of saponification of the partially saponified polyvinyl alcohol can be 75 mol% or more and less than 98 mol%, and may be 80 to 95 mol%. When the degree of saponification of the partially saponified polyvinyl alcohol is 75 mol% or more, sufficient barrier properties are easily obtained, and when it is less than 98 mol%, good wettability is easily obtained.

The degree of polymerization of polyvinyl alcohol (partially saponified and completely saponified polyvinyl alcohol) is desirably in the range of 300 to 3,000, or 300 or more and less than 3,000. If the degree of polymerization is less than 300, the barrier property is likely to decrease, and if it is more than 3,000, the viscosity is too high and the coating suitability is likely to decrease. From this viewpoint, the degree of polymerization may be 300 to 1,500.

(Polyethyleneimine)
Polyethyleneimine is a copolymer of ethyleneimine, and includes linear ones containing only secondary amines, and branched ones containing primary, secondary, and tertiary amines, but either one may be used. A mixture of multiple types may also be used. A derivative having a functional group such as a cationic group introduced therein may also be used.

The weight-average molecular weight of the polyethyleneimine is desirably 500 or more, or 1500 or more. Polyethyleneimine with a weight-average molecular weight that is too small tends to penetrate into the membrane, and there is a risk that sufficient water resistance will not be achieved. The upper limit of the weight-average molecular weight can be 100,000.

<Method of manufacturing water-resistant gas barrier film>
The method for producing a water-resistant gas barrier film includes a step of applying a coating liquid containing (a) at least one of carboxy-modified polyvinyl alcohol and partially saponified polyvinyl alcohol, (b) fully saponified polyvinyl alcohol, and (c) polyethyleneimine onto a substrate film (coating step), and a step of heating and drying the applied coating liquid to form a coating film on the substrate film (heat drying step). The production method may further include a step of stretching the substrate film together with the coating film (stretching step).

(Coating process)
The powders of the carboxy-modified polyvinyl alcohol, the partially saponified polyvinyl alcohol, and the fully saponified polyvinyl alcohol are weighed in the required amounts and dissolved in water heated to 80° C. or higher. An aqueous solution in which each powder is individually dissolved may be prepared in advance. The amount of solids in the aqueous solution is not particularly important, but it is preferable for handling that the amount of solids is less than 20% by mass. From the viewpoint of coating efficiency, etc., the lower limit of the solids amount may be 1% by mass.

A coating liquid is obtained by further adding polyethyleneimine to the aqueous solution containing polyvinyl alcohol. If necessary, the coating liquid may contain preservatives, alcohol, leveling agents, antifoaming agents, ultraviolet absorbers, antioxidants, silane coupling agents, metal chelating agents, etc.

When at least one of carboxy-modified polyvinyl alcohol and partially saponified polyvinyl alcohol is the component (a), fully saponified polyvinyl alcohol is the component (b), and polyethyleneimine is the component (c), the quantitative ratio of the components (a) to (c) in the coating liquid may be appropriately adjusted so that the quantitative ratio of each component in the coating film becomes, for example, a desired value described later.

The coating step can be carried out by a coating method using an aqueous solution. As the coating method, a known method can be used, and specifically, a wet film formation method using a gravure coater, a dip coater, a reverse coater, a wire bar coater, a die coater, or the like can be mentioned.

Prior to the coating step, a pre-stretching step may be carried out to pre-stretch the substrate film. In particular, when a sequentially stretched film is to be obtained, the substrate film is stretched in only one axial direction (e.g., MD direction) in the pre-stretching step. The stretching ratio may be 2 to 20 times.

(Heat drying process)
The drying temperature in the heat drying step is preferably 90°C or higher. There is no problem if the drying temperature is less than 250°C. The dry film thickness after coating is preferably 0.01 to 20 μm. The thinner the film, the faster the processing speed will be, and the thicker the film, the better the barrier properties will be. Appropriate adjustments may be made as necessary.

When at least one of carboxy-modified polyvinyl alcohol and partially saponified polyvinyl alcohol is component (a), fully saponified polyvinyl alcohol is component (b), and polyethyleneimine is component (c), it is preferable to adjust the amount of each component in the coating film as follows.
In the formed coating film, the total amount of components (a) and (b) is preferably 90 to 99.9 mass %, and more preferably 95 to 99.5 mass %, based on the total amount of the coating film. The ratio of the amount of component (a) to the total amount of components (a) and (b) (amount of component (a)/total amount of components (a) and (b)) is preferably 0.01 to 0.5, and more preferably 0.02 to 0.4. This makes it easier to maintain good barrier properties and wettability.

In the formed coating film, the content of component (c) is desirably 10% by mass or less, or 5% by mass or less, based on the total amount of the coating film. Adding too much leads to a decrease in barrier properties. The lower limit of the content of component (c) can be 0.01% by mass.

(Stretching process)
Since the coating has sufficient flexibility, the base film can be stretched together with the coating in this step. The stretching is performed to control the heat shrinkage properties. In the stretching step, the base film is stretched to a predetermined ratio at a temperature equal to or higher than its Tg. The stretching ratio can be 2 to 20 times, and may be 3 to 10 times. If the stretching ratio is less than 2 times, the shrinkage properties do not change much, and if it is more than 20 times, some films may break. In this manufacturing method, since the base film can be stretched after the coating is formed, in-line barrier coating is possible. Therefore, the above manufacturing method can provide a water-resistant gas barrier film at low cost.

The water-resistant gas barrier film obtained by the above-mentioned production method may be obtained by forming a coating film on a substrate film that has been pre-stretched in one axis direction (e.g., MD direction) as necessary, and then further stretching the substrate in another axis direction (e.g., TD direction). The film obtained by such a process is a film that has been stretched successively, not simultaneously in two axes, and therefore can be called a successively stretched film.

After the stretching step, a post-heating step may be carried out, if necessary.

The present invention will be described in more detail below with reference to examples (Example 3 is a reference example) , but the present invention is not limited to these examples.

<Film Preparation>
[(a1) Preparation of carboxy-modified polyvinyl alcohol aqueous solution]
Vinyl acetate and maleic acid were polymerized using 2,2-azobisisobutyronitrile as a catalyst. The polymer was saponified with an aqueous sodium hydroxide solution (containing methanol) and then dried at 80°C to obtain (a) a carboxy-modified polyvinyl alcohol powder. The powder was dissolved in water at 90°C.

[(a2) Preparation of partially saponified aqueous polyvinyl alcohol solution]
Vinyl acetate was polymerized using 2,2-azobisisobutyronitrile as a catalyst. The polymer was partially saponified with an aqueous sodium hydroxide solution (containing methanol) and then dried at 80°C to obtain (a) partially saponified polyvinyl alcohol powder with a polymerization degree of 1000. The saponification degree was 87 mol%. The powder was dissolved in water at 90°C.

(b) Preparation of fully saponified aqueous polyvinyl alcohol solution
Vinyl acetate was polymerized using 2,2-azobisisobutyronitrile as a catalyst. The polymer was saponified with an aqueous sodium hydroxide solution (containing methanol) and then dried at 80°C to obtain (b) a fully saponified polyvinyl alcohol powder with a degree of polymerization of 500. The degree of saponification was 98 mol% or more. The powder was dissolved in water at 90°C.

(c) Preparation of Polyethylenimine
Several types of polyethyleneimine having different weight average molecular weights were prepared.

[Preparation of Coating Fluid]
Various aqueous polyvinyl alcohol solutions and polyethyleneimine were weighed out so that the amounts of various polyvinyl alcohols and polyethyleneimine were in the ratios shown in Table 1, and these were mixed for 10 minutes or more. In this way, coating liquids used in each example were obtained. The total content (solid content) of (a1) carboxy-modified polyvinyl alcohol or (a2) partially saponified polyvinyl alcohol and (b) fully saponified polyvinyl alcohol in the coating liquid was adjusted to 10 mass% based on the total amount of the coating liquid.

Example 1
(a1) Carboxy-modified polyvinyl alcohol and (b) fully saponified polyvinyl alcohol were mixed in a mass ratio of about 1:2, and polyethyleneimine having a weight average molecular weight of 1800 was added thereto to prepare a coating liquid. The amount of polyethyleneimine added was adjusted so that the content in the coating film was 2 mass%. This coating liquid was applied by a bar coating method onto a polyethylene terephthalate film substrate (P60, manufactured by Toray Industries, Inc.) having a thickness of 12 μm. The coating liquid was then dried and cured at 150° C. for 3 minutes to form a coating film having a thickness of 200 nm on the substrate.

Example 2
(a1) Carboxy-modified polyvinyl alcohol and (b) fully saponified polyvinyl alcohol were mixed in a mass ratio of about 1:2, and polyethyleneimine having a weight average molecular weight of 10,000 was added thereto to prepare a coating solution. The amount of polyethyleneimine added was adjusted so that the content in the coating film was 1 mass%. Thereafter, a coating film was formed on the substrate in the same manner as in Example 1.

Example 3
A coating film was formed on a substrate in the same manner as in Example 2, except that (a2) partially saponified polyvinyl alcohol was used instead of (a1) carboxy-modified polyvinyl alcohol.

Example 4
A coating film was formed on the substrate in the same manner as in Example 2, except that a 20 μm-thick unstretched polypropylene film substrate was used instead of the 12 μm-thick polyethylene terephthalate film substrate. Then, the unstretched polypropylene film substrate was stretched in the TD direction together with the coating film. The stretching ratio was 3 times.

Example 5
A coating film was formed on a substrate in the same manner as in Example 1, except that a polyethyleneimine having a weight average molecular weight of 10,000 was used instead of a polyethyleneimine having a weight average molecular weight of 1,800, and a non-stretched polyethylene terephthalate film substrate having a thickness of 150 μm was used instead of a polyethylene terephthalate film substrate having a thickness of 12 μm. Then, the non-stretched polyethylene terephthalate film substrate was stretched in the TD direction together with the coating film. The stretching ratio was 3 times.

Example 6
A coating film was formed on a substrate in the same manner as in Example 1, except that polyethyleneimine having a weight-average molecular weight of 600 was used instead of polyethyleneimine having a weight-average molecular weight of 1,800.

(Example 7)
A coating film was formed on a substrate in the same manner as in Example 2, except that the amount of polyethyleneimine added was adjusted so that the content in the coating film was 10 mass %.

(Comparative Example 1)
(b) A coating film was formed on a substrate in the same manner as in Example 1, except that a coating liquid containing only fully saponified polyvinyl alcohol was used.

<Film performance evaluation>
The performance of the film was evaluated according to the following methods, and the results are shown in Table 2.
[Water-resistant adhesion]
The coating surface of the prepared film was laminated to a CPP film via an adhesive, and then cut into a 15 mm x 100 mm sample. After exposing the peeled surface, the sample was immersed in water and the laminate strength was measured according to JIS Z1707. The measurement conditions were a peel speed of 300 m/min and a peel angle of 180 degrees. If the laminate strength was 1.0 N or more, the water-resistant adhesion was evaluated as good.

[Oxygen permeability]
The oxygen permeability of the prepared film was measured under conditions of 30°C and 70% RH by differential pressure gas chromatography in accordance with JIS-K7129 Method C. If the oxygen permeability is 50 (cc/ ^m2 ·day·atm) or less, the barrier property can be evaluated as good, and if the oxygen permeability is 15 (cc/ ^m2 ·day·atm) or less, the barrier property can be evaluated as particularly good.

As shown in Table 2, the films of the examples were good in both water-resistant adhesion and barrier property, whereas the films of the comparative examples were good in barrier property but poor in water-resistant adhesion.

From the above results, it was found that the water-resistant gas barrier film of the present invention has good water-resistant adhesion and excellent barrier properties.

Reference Signs List 1: base film, 2: gas barrier layer, 10: water-resistant gas barrier film.

Claims (9)

A gas barrier film having at least a gas barrier layer provided on a base film,
The gas barrier layer is a water-resistant gas barrier film formed from a coating liquid containing (a) carboxy-modified polyvinyl alcohol, (b) fully saponified polyvinyl alcohol, and (c) polyethyleneimine. 2. The water-resistant gas barrier film according to claim 1, wherein the fully saponified polyvinyl alcohol has a degree of polymerization of 3,000 or less. The water-resistant gas barrier film according to claim 1 or 2, wherein the content of the polyethyleneimine is 10% by mass or less based on the total amount of the gas barrier layer. The water-resistant gas barrier film according to any one of claims 1 to 3, wherein the weight average molecular weight of the polyethyleneimine is 500 or more. The water-resistant gas barrier film according to any one of claims 1 to 4, wherein the carboxy-modified polyvinyl alcohol is a saponified copolymer of a vinyl ester monomer and a carboxy-containing monomer. The water-resistant gas barrier film according to any one of claims 1 to 5, which is a sequentially stretched film. A step of applying a coating liquid containing (a) carboxy-modified polyvinyl alcohol, (b) fully saponified polyvinyl alcohol, and (c) polyethyleneimine onto a substrate film;
a step of heating and drying the applied coating liquid to form a coating film on the substrate film;
The method for producing a water-resistant gas barrier film comprises: The method of claim 7 further comprises a step of stretching the base film together with the coating film. The method of claim 8, wherein the stretching ratio in the stretching step is 2 to 20 times.

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